1. Technical Field
The invention relates to communications protocols and, more particularly, to a memory structure for use by a terminal device while exchanging messages with a transaction processor.
2. Description of the Prior Art
A host computer (processor) that communicates with different types of computer terminals and devices usually customizes a command so that the command is in a form expected by the terminal that is currently communicating with the host. If a command is not so customized, then the terminal might possibly execute a process in a way not intended by the host. For example, if the command pertains to displaying a particular pattern, then the terminal might possibly display some variation of that pattern.
To deal with that possibility, the host initially requests the identity (model) of a computer terminal as a way of identifying the characteristics defining the display associated with the computer terminal. Such characteristics include, e.g., the size, shape, aspect ratio and resolution of the terminal display. Once it knows such characteristics, the host may then properly format a display command so that it suits the computer terminal. One such command includes the location at which a pattern, or symbol, is to be displayed on the terminal display. The host specifies such a location so that it may maintain in its internal memory, a map of the locations at which symbols are to be displayed on the terminal display.
For example, one display pattern that a host computer typically transmits to a terminal is a so-called menu defining a list of selectable items (e.g., services). A user operating the terminal may select one of the displayed menu items using any one of a number of different input devices. One such input device is a conventional computer keyboard associated with a displayed screen cursor. To select a desired menu item, the user moves the screen cursor in a conventional manner to the displayed item and then operates an appropriate keyboard button, e.g., the Enter key. However, the movement of the displayed cursor is typically under the control of the host computer. That is, if the user moves the screen cursor in a particular direction (up, down, left or right), then the associated computer terminal sends a message indicative of that fact to the host computer. The host computer, in response thereto, returns a message directing the terminal to move the screen cursor a number of units in the identified direction. The host computer also tracks the new display location of the screen cursor in its memory map.
Once the screen cursor is positioned on the desired menu item, the user may then operate the Enter key. The user's terminal in response thereto notifies the host computer that the user operated the Enter key. The host computer, in turn, correlates the operation of the Enter key with the position of the screen cursor using its stored map of the display, and therefore determines that the screen cursor is positioned on a menu item. Accordingly, the host computer interprets the message as a request to invoke the selected menu item. A terminal device and host computer may reduce the level of communications that they exchange during a transaction session by using what is commonly referred to as a "block mode". In the block mode, the terminal accumulates the user's input until the user operates a transmit key. At that point, the terminal device transmits the user's accumulated input to the host processor. However in either case, the host needs to obtain precise knowledge of the terminal display, especially the size of the display, in order to properly format the presentation of information to a user.